I think the purpose of the metaphor is that external activity and internal state are non-correlated.
With great intensity and interest chopping wood (or coding or whatever) can be really fascinating. Without interest most activities are chores.
After enlightenment; chop wood, haul water.
... Does this really apply now when vanishingly few people do either of these ever, much less daily?
the "Mexican Fisherman" story fits in here too.